The present invention relates to automatic hook-up of a hose-and-drogue aerial refueling apparatus to a receiver aircraft probe and, more particularly, to apparatus and methods for in-flight hose-and-drogue refueling using electrooptical technology.
The usefulness of air refueling became apparent to the military almost as soon as they started using aircraft. The main advantage of air refueling is obvious: it enables aircraft to stay airborne longer. Since most aircraft are incapable of taking off with maximum fuel and full payload, without in-flight refueling there is always a balance to be struck between range, payload, and fuel. Air refueling is more than just a range stretcher: it allows one to carry out missions with a smaller number of sorties, or alternatively, fewer aircraft.
One approach to in-flight refueling is the hose-and-drogue system, in which a drogue attached to a fuel hose is extended from the refueling aircraft's belly or wings. The receiver aircraft is equipped with a fixed or retractable probe and the receiving aircraft's pilot flies the probe into the drogue.
The advantages of the hose-and-drogue system are the following: (a) up to three receivers can take fuel simultaneously; (b) if one hose/drogue unit (HDU) becomes unserviceable, the tanker still can offload its fuel; (c) the HDU is inherently safer than the heavy, rigid boom, which is restricted in its movements; (d) it is easier to install on non-purpose-built aircraft; and (e) it is compatible with most receivers, e.g., fixed wings, as well as rotorcraft.
There were, however, two disadvantages to early hose and drogue system: it has lower fuel transfer rates than the boom system, and the drogue is uncontrollable and is susceptible to winds and gusts. In bad weather conditions and particularly in low level refueling situations the hookup process was very difficult and demanded excessive receiving aircraft pilot maneuvers. Since the aircraft &o be refueled was likely to be already low on fuel, excessive maneuvering could result in the necessity to abandon the aircraft.
U.S. Pat. No. 5,326,052, which issued to Krispin et al. on Jul. 5, 1994 (hereinafter the '052 patent), and the entire disclosure of which is incorporated herein by reference, discloses a system designed to overcome the problem of drogue instability by providing a system with means for both drogue steering and drogue motion measurement, thus enabling either stabilization of the drogue or, alternatively, fully automatic hookup.
Specifically, the system disclosed in the '052 patent includes a plurality of miniature, pressurized gas thrusters mounted, preferably equally spaced, on the perimeter of a drogue in such way that activation of any thruster generates a force in a direction perpendicular to the fuel outlet nozzle. In principle, four thrusters are sufficient. Two are required to control the drogue in up-and-down motion and two in the sideways motion. The position of the drogue is measured by electrooptical position-sensing devices.
The measurement system consists of a plurality of light sources (LS) such as Light Emitting Diodes (LEDs) or Laser Diodes (LDs) mounted at a certain distance from the end of the receiving fuel probe, a plurality of sensors each comprising a lens and a position-measuring photodetector that is mounted on the perimeter of the drogue with its sensitive area directed toward the receiving aircraft, and a processing electronic circuit for calculating the position of the light-source assembly relative to the detector assembly and computing the control commands to the thrusters.
In accordance with one preferred form of the invention disclosed in the '052 patent, the measured position of the drogue relative to the probe of the receiving aircraft is used to determine the control commands to the thrusters so that the drogue will track the position of the probe in order to achieve automatic hookup. In such an arrangement the pilot of the receiver aircraft is required to approach the vicinity of the tanker aircraft; once the two aircraft have closed to a certain distance, the automatic control system of the drogue is activated and guides the drogue until contact with the incoming probe has been achieved.
In accordance with an alternative form of the invention disclosed in the '052 patent, measurements are made of the drogue position relative to the tanker aircraft, either belly or wings, and are used to control the motion of the drogue so as to stabilize or, equivalently, to minimize the motion of the drogue relative to the tanker aircraft. In such an arrangement, the pilot of the receiver aircraft is required to track the drogue, which has been stabilized relative to the tanker. The tanker is much less susceptible to wind gusts and turbulence.